This invention relates to evaluation of geological information and more particularly relates to analysis of geophysical data to determine the porosity of a geologic formation.
Fracture detection in coal seams is critical for the recovery of economic quantities of methane. Coal is a dual porosity medium, comprising a matrix containing abundant micro-scale pores intersected by larger macro-scale fractures. The micro-scale pores are of the size that gas movement occurs via diffusion, resulting in a very slow rate of gas exchange per unit volume. The larger macro-scale fractures act as the conduits for connecting the gas-diffusing matrix to a well bore. For economic quantities of methane to be recoverable, extensive well-developed macro-scale fractures must be present to connect a large enough volume of coal matrix such that the total volume of gas diffused becomes significant. Thus, the detection of subsurface fracture systems is critical for delineating desirable locations for methane exploration.
During the drilling of a wellbore, drilling mud is pumped into the well in order to flush rock chips and other unwanted debris from the well bore as it is being drilled. The drilling mud is introduced into the wellbore under pressure, where the mud pressure is slightly greater than the pressure of a formation traversed by the wellbore to prevent escape of material from the formation through the well bore, a phenomenon known as well blowout. The resultant differential pressure between the mud well bore column pressure and the formation pressure forces mud filtrate into the permeable formation, and solid particles of the mud are deposited on the wellbore wall, forming a mudcake.
The mudcake usually has a very low permeability, and once developed, considerably reduces the rate of further mud filtrate invasion into the wellbore wall. In a region very close to the wellbore wall, most of the original formation may be flushed away by the mud filtrate. This region is known as the xe2x80x9cflushed zonexe2x80x9d or the xe2x80x9cinvaded zonexe2x80x9d. If the flushing is complete, the flushed zone pore space contains only mud filtrate.
Further out from the wellbore wall, the displacement of the formation fluids by the mud filtrate is less and less complete. This results in a second region, this region undergoing a transition from mud filtrate saturation to original formation water saturation. The second region is known as the xe2x80x9ctransition zonexe2x80x9d. The extent or depth of the flushed and transition zones depends on many parameters. Among them is the type and characteristics of the drilling mud, the formation porosity, the formation permeability, the pressure differential and the time since the well was first drilled. The undisturbed formation beyond the transition zone is known as the xe2x80x9cuninvaded, virgin or uncontaminated zonexe2x80x9d.
FIGS. 1 and 2 show prior art representations of an invasion and resistivity profile in a water-bearing zone. FIG. 1 illustrates a cross section of a wellbore showing the locations of the mud cake 8 formed on the inner surface of the well bore by the drilling mud, the flushed zone, the transition zone and the uninvaded zone extending radially from the wellbore wall. FIG. 2 illustrates the formation of a mud cake 8 by the drilling mud along the well bore wall and a radial distribution of formation resistivity extending radially from the wellbore wall, into the flushed zone, into the transition zone, and into the uninvaded zone. Sometimes, in oil and gas bearing formations, where the mobility of the hydrocarbons is greater than that of the water, because of relative permeability differences, the oil or gas moves away faster than the interstitial water. In this case, there may be formed, between the flushed zone and the uninvaded zone, an xe2x80x9cannular zone or annulusxe2x80x9d, with a high formation water saturation. Annuli probably occur in most hydrocarbon bearing formations; and their influence on measurement depends on the radial location of the annulus and its severity.
The existence of these zones (the flushed, transition, annular and uninvaded zones) influence resistivity log measurements and therefore the accuracy of the resistivity log itself. In it""s conventional use, the resistivity log is used to determine if oil exists in the formation traversed by the wellbore. The main interest of the resistivity log, shown in the upper portion 10 of the graph of FIG. 2, is to obtain the true and correct value of the resistivity (reciprocal of the conductivity) of the uninvaded zone, Rt in the graph. High values of Rt indicate the presence of an insulator, possibly oil, in the formation. Conventionally, it is therefore desirable to correct for the effect of mud filtrate invasion on formation resistivity.
Conventionally, mud filtrate invasion analysis from resistivity logs is attempted by qualitative inspection of the separation between measurement displays representing different depths of investigation. The purpose of this analysis is to determine the radial geometric function of the logging tool response in order to correct for invasion and generate a more accurate value of Rt. It is desirable to have a method for determining geological formation fracture porosity that does not rely on values of Rt.
Conventional log analysis techniques require correction for hydrocarbon saturation in the void spaces of the geologic formation, and are complicated by depth based variation in the hydrocarbon saturation gradient through the flushed zone/uninvaded zone interface that may confuse invasion character. Variations in drilling mud properties between wells that change the radial resistivity profile and differences in the properties of the formation water can cause errors in conventional interpretation. As well, laboratory measurements of fracture porosity in coal may not be applicable to the bulk reservoir properties due to sampling error, the inherent friability of coal, and the sensitivity of coal to changes in stress regime. It is desirable to have a basis for analysis that does not require correction for hydrocarbon saturation or depth based variations.
After an exploration well is drilled, specialized tools are lowered down into the bore hole to test and record the responses of the different rock formations to various electrical, acoustic and radioactive stimuli. This process is termed geophysical logging, and the recorded data are termed geophysical logs. In one petroleum producing region of the world, the Western Canada Sedimentary Basin, approximately 280,000 wells have been drilled to date, and geophysical logs exist for virtually all of them. Geophysical logs have been used extensively in the past in conventional oil and gas exploration, but little data exist on their use in fracture detection in coal.
Some highly specialized geophysical logs are able to detect fractures in coal under very specific conditions, but the data are prone to error and the logging techniques have seen limited use. Advancement in the art delineated by the disclosed invention is that a large portion of previously unused data can now be processed for a new and useful result.
Various geophysical techniques exist to detect mud filtrate invasion and/or mudcake. One such device is an electrical pad containing regularly spaced electrodes. As the pad moves across the target formation, variations between the voltages are recorded, detecting the existence of mudcake on the borehole wall. This device relies on a solid contact with the bore hole wall and any variations in the size of the hole can disrupt its operation. This is significant as, over time, coals tend to cave-in resulting in rugose and irregular bore holes, thus limiting the utility of the pad contact type device.
Other types of electrical logging devices exist, but all have the goal of determining the rock properties away from the invasive and damaging effects of the well bore. In general, most of these devices are able to detect the depth accurately that the drilling fluid has invaded. However, because of the complex geometry of the pore spaces in conventional clastic and carbonate reservoir rocks and variation in hydrocarbon saturation, invasion has not been previously considered a quantifiable indicator of porosity.
U.S. Pat. No. 5,663,499 discloses a method for estimating permeability using geophysical well log data. This method interprets data from a multi-array induction device having at least five resistivity measures for a given formation and uses a variety of complex estimates, measurements and calculations. The measurements required include estimates of gas gravity, cementation factor, saturation exponent, shale volume and, and many others. The method requires a specialized logging apparatus to generate the required data and is unable to examine pre-existing data.
European patent EP0363259 discloses a method for interpreting data from a formation micro-scanner, a pad contact type of device, to detect and estimate width of fractures intersecting a borehole. It is limited in use and unable to examine pre-existing data.
U.S. Pat. No. 5,379,216 discloses a method and a highly specialized apparatus for measuring invading volumes of mud filtrate to determine relative measurements of permeability. However, this patent is limited to analysis of data generated by its own disclosed apparatus, and is unable to analyze pre-existing data for indications of fracture porosity.
U.S. Pat. No. 4,961,343 discloses a method for determining permeability of a subsurface earth formation in real time during drilling operations through monitoring volumes of drill fluid lost into the formation and volumes of gas liberated. Geophysical log responses are not used. As well, this patent is limited in utility as no means of examining pre-existing data is disclosed.
This invention relates to a method for determining fracture porosity in coals using existing well bore induction logging data produced by an induction tool disposed in the well bore. The disclosed invention uses conventional well log data in an unconventional manner to determine new and useful information regarding well bore formation properties, specifically the amount of fracture porosity.
The invention provides a method of analysis to determine geologic formation fracture porosity that does not rely on formation resistivity, Rt.
The disclosed invention is volume based and requires no correction for hydrocarbon saturation or depth based variations. As all effectively connected fractures in coal are filled with water, hydrocarbon saturation variations are immaterial. Variations in mud properties are screened out or are of no impact to the disclosed invention, as the true value of Rt is irrelevant. As well, data used in the disclosed invention are collected from the formation in situ, with the coals under actual temperature and pressure conditions and are more representative of the bulk reservoir properties. In accordance with the present invention, fracture porosity calculations in coal are performed in the volume domain. Use of a volume domain mud filtrate invasion analysis minimizes the effect of all of these variables and is useful for comparing well to well and between zones within a well for determining measures of fracture porosity, and hence, methane production potential in the coal seams.
This invention relates methods of detecting fracturing in rock using geophysical logs, and in particular the geophysical log data produced by electrical type logging devices. The disclosed invention seeks to remedy these deficiencies in the prior art of fracture detection in coal through a method that incorporates data previously unused for determination of formation fracture porosity into a new and useful result. Coals are uniquely suited to this method, as the fractures tend to occur in a regularly spaced orthogonal geometry. This type and pattern of fracturing simplifies the determination of invading and invaded volumes.
As well, coals comprise a special case where only fractures that are effectively connected to the borehole are available to invasion of drilling fluids. Fracture porosity is then directly related to the volume of coal effectively connected for gas diffusion, and therefore, is a major indicator of economic methane production. The disclosed invention represents a significant advancement in the art as previously by-passed reservoirs of methane can now be found.
The disclosed invention screens existing geophysical well logs to ensure reliable data by discarding wells where the resistivity of the drilling mud (Rm) is less than 1.0. Experience has shown that below this value, induction logs are affected by the conductivity of the drilling mud and unreliable values of depth of invasion are produced. A second screening procedure involves the examination of the borehole caliper log. This log measures the size of the borehole. Measurements of the borehole diameter that exceed 200% of the bit size are considered unreliable and screened out.
Measurements of the thickness of the coal seam of interest, the bit size and the depth of invasion of drilling fluids define an invaded volume of coal.
From records of the characteristics of the drilling fluid, a measure of the amount of fluid available to create this invasion can be made. The volume of fluid available for invasion is then divided by the volume of the invaded rock. The resulting volume fraction equals the effective void space occupied by the invading fluid.
In coal, this volume fraction of effective void space is fracture porosity, as only fractures are able to accept invading fluids; as the coal of the formation matrix is impermeable. The disclosed invention outlines a new, useful and unconventional method for interpreting previously unused data and delineating methane exploration targets.
In one of its aspects, the invention provides a method for identifying fractures in coal seams using geophysical log records recording measurements of well data including induction type geophysical data relating to a well bore comprising providing at least one geophysical log record recording measurements of well data including data induction type geophysical data, selecting log records acceptable for use and calculating a formation porosity.
In another of its aspects, the invention provides a method for identifying fractures in coal seams using measurements of drilling fluid resistivity, drilling fluid loss, surface area of the filter used to measure drilling fluid loss, well bore hole diameter, drill bit size, thickness of the coal seam, measurements of shallow, medium and deep resistivities of the coal seams taken from geophysical logs, said method comprising a first screening step selecting only geophysical logs exhibiting resistivity of drilling mud greater than 1.0 ohm-meter. These geophysical logs are then screen for a bore hole caliper size through a target zone of each said selected geophysical is less than 200% of bit size.
For each screened, selected geophysical log, the following steps are performed, namely, calculating a first ratio from a value of medium resistivity derived from the data of said selected geophysical log divided by a value of deep resistivity derived from the data of said selected geophysical log. Then a second ratio is calculated from a value of shallow resistivity derived from the data of said selected geophysical log divided by said value of deep resistivity. A depth of invasion is determined from the first and second ratios. A fluid affected volume is calculated from the square of the sum of a radius of a well bore bit size plus the depth of invasion, the square of the sum being multiplied by pi and by a thickness of the target zone. A volume of invaded coal is calculated from the fluid affected volume less the product of the square of said radius of a well bore bit size, times pi, times the thickness of the target zone. A volume of invading fluid is calculated from a drilling fluid loss measurement of the selected geophysical log well log and corrected for time and filter surface area. A fracture porosity is calculated as the ratio of said volume of invading fluid divided by said volume of invaded coal.